After completing NG+, you will be able to go to a third playthrough, which doesn't add anything but an optional superboss. This boss will be found via one of four different portals throughout the overworld, where it will suck in a random character for a one-on-one fight. Defeat the boss and that character will gain access to their ultimate weapon.
These four locations are as follows:
- Black Tortoise Crevasse (near giant crater)
- Near Militesi Capital in Azurr region
- Near royal palace in Rilochy Province
- Innsmouth region